mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-25 12:12:47 +01:00
368 lines
15 KiB
HTML
368 lines
15 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
<html>
|
|
<head>
|
|
<title>LLVM Link Time Optimization: Design and Implementation</title>
|
|
<link rel="stylesheet" href="llvm.css" type="text/css">
|
|
</head>
|
|
|
|
<div class="doc_title">
|
|
LLVM Link Time Optimization: Design and Implementation
|
|
</div>
|
|
|
|
<ul>
|
|
<li><a href="#desc">Description</a></li>
|
|
<li><a href="#design">Design Philosophy</a>
|
|
<ul>
|
|
<li><a href="#example1">Example of link time optimization</a></li>
|
|
<li><a href="#alternative_approaches">Alternative Approaches</a></li>
|
|
</ul></li>
|
|
<li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
|
|
<ul>
|
|
<li><a href="#phase1">Phase 1 : Read LLVM Bytecode Files</a></li>
|
|
<li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
|
|
<li><a href="#phase3">Phase 3 : Optimize Bytecode Files</a></li>
|
|
<li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
|
|
</ul></li>
|
|
<li><a href="#lto">LLVMlto</a>
|
|
<ul>
|
|
<li><a href="#llvmsymbol">LLVMSymbol</a></li>
|
|
<li><a href="#readllvmobjectfile">readLLVMObjectFile()</a></li>
|
|
<li><a href="#optimizemodules">optimizeModules()</a></li>
|
|
<li><a href="#gettargettriple">getTargetTriple()</a></li>
|
|
</ul></li>
|
|
<li><a href="#debug">Debugging Information</a></li>
|
|
</ul>
|
|
|
|
<div class="doc_author">
|
|
<p>Written by Devang Patel</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="desc">Description</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
LLVM features powerful intermodular optimizations which can be used at link
|
|
time. Link Time Optimization is another name for intermodular optimization
|
|
when performed during the link stage. This document describes the interface
|
|
and design between the LLVM intermodular optimizer and the linker.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="design">Design Philosophy</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
The LLVM Link Time Optimizer provides complete transparency, while doing
|
|
intermodular optimization, in the compiler tool chain. Its main goal is to let
|
|
the developer take advantage of intermodular optimizations without making any
|
|
significant changes to the developer's makefiles or build system. This is
|
|
achieved through tight integration with the linker. In this model, the linker
|
|
treates LLVM bytecode files like native object files and allows mixing and
|
|
matching among them. The linker uses <a href="#lto">LLVMlto</a>, a dynamically
|
|
loaded library, to handle LLVM bytecode files. This tight integration between
|
|
the linker and LLVM optimizer helps to do optimizations that are not possible
|
|
in other models. The linker input allows the optimizer to avoid relying on
|
|
conservative escape analysis.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="example1">Example of link time optimization</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>The following example illustrates the advantages of LTO's integrated
|
|
approach and clean interface.</p>
|
|
<ul>
|
|
<li> Input source file <tt>a.c</tt> is compiled into LLVM byte code form.
|
|
<li> Input source file <tt>main.c</tt> is compiled into native object code.
|
|
</ul>
|
|
<pre>
|
|
--- a.h ---
|
|
extern int foo1(void);
|
|
extern void foo2(void);
|
|
extern void foo4(void);
|
|
--- a.c ---
|
|
#include "a.h"
|
|
|
|
static signed int i = 0;
|
|
|
|
void foo2(void) {
|
|
i = -1;
|
|
}
|
|
|
|
static int foo3() {
|
|
foo4();
|
|
return 10;
|
|
}
|
|
|
|
int foo1(void) {
|
|
int data = 0;
|
|
|
|
if (i < 0) { data = foo3(); }
|
|
|
|
data = data + 42;
|
|
return data;
|
|
}
|
|
|
|
--- main.c ---
|
|
#include <stdio.h>
|
|
#include "a.h"
|
|
|
|
void foo4(void) {
|
|
printf ("Hi\n");
|
|
}
|
|
|
|
int main() {
|
|
return foo1();
|
|
}
|
|
|
|
--- command lines ---
|
|
$ llvm-gcc4 --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bytecode file
|
|
$ llvm-gcc4 -c main.c -o main.o # <-- main.o is native object file
|
|
$ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifications
|
|
</pre>
|
|
<p>In this example, the linker recognizes that <tt>foo2()</tt> is an
|
|
externally visible symbol defined in LLVM byte code file. This information
|
|
is collected using <a href="#readllvmobjectfile"> readLLVMObjectFile()</a>.
|
|
Based on this information, the linker completes its usual symbol resolution
|
|
pass and finds that <tt>foo2()</tt> is not used anywhere. This information
|
|
is used by the LLVM optimizer and it removes <tt>foo2()</tt>. As soon as
|
|
<tt>foo2()</tt> is removed, the optimizer recognizes that condition
|
|
<tt>i < 0</tt> is always false, which means <tt>foo3()</tt> is never
|
|
used. Hence, the optimizer removes <tt>foo3()</tt>, also. And this in turn,
|
|
enables linker to remove <tt>foo4()</tt>. This example illustrates the
|
|
advantage of tight integration with the linker. Here, the optimizer can not
|
|
remove <tt>foo3()</tt> without the linker's input.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="alternative_approaches">Alternative Approaches</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<dl>
|
|
<dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
|
|
<dd>In this model the link time optimizer is not able to take advantage of
|
|
information collected during the linker's normal symbol resolution phase.
|
|
In the above example, the optimizer can not remove <tt>foo2()</tt> without
|
|
the linker's input because it is externally visible. This in turn prohibits
|
|
the optimizer from removing <tt>foo3()</tt>.</dd>
|
|
<dt><b>Use separate tool to collect symbol information from all object
|
|
files.</b></dt>
|
|
<dd>In this model, a new, separate, tool or library replicates the linker's
|
|
capability to collect information for link time optimization. Not only is
|
|
this code duplication difficult to justify, but it also has several other
|
|
disadvantages. For example, the linking semantics and the features
|
|
provided by the linker on various platform are not unique. This means,
|
|
this new tool needs to support all such features and platforms in one
|
|
super tool or a separate tool per platform is required. This increases
|
|
maintance cost for link time optimizer significantly, which is not
|
|
necessary. This approach also requires staying synchronized with linker
|
|
developements on various platforms, which is not the main focus of the link
|
|
time optimizer. Finally, this approach increases end user's build time due
|
|
to the duplication of work done by this separate tool and the linker itself.
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="multiphase">Multi-phase communication between LLVM and linker</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>The linker collects information about symbol defininitions and uses in
|
|
various link objects which is more accurate than any information collected
|
|
by other tools during typical build cycles. The linker collects this
|
|
information by looking at the definitions and uses of symbols in native .o
|
|
files and using symbol visibility information. The linker also uses
|
|
user-supplied information, such as a list of exported symbols. LLVM
|
|
optimizer collects control flow information, data flow information and knows
|
|
much more about program structure from the optimizer's point of view.
|
|
Our goal is to take advantage of tight intergration between the linker and
|
|
the optimizer by sharing this information during various linking phases.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="phase1">Phase 1 : Read LLVM Bytecode Files</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>The linker first reads all object files in natural order and collects
|
|
symbol information. This includes native object files as well as LLVM byte
|
|
code files. In this phase, the linker uses
|
|
<a href="#readllvmobjectfile"> readLLVMObjectFile() </a> to collect symbol
|
|
information from each LLVM bytecode files and updates its internal global
|
|
symbol table accordingly. The intent of this interface is to avoid overhead
|
|
in the non LLVM case, where all input object files are native object files,
|
|
by putting this code in the error path of the linker. When the linker sees
|
|
the first llvm .o file, it <tt>dlopen()</tt>s the dynamic library. This is
|
|
to allow changes to the LLVM LTO code without relinking the linker.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="phase2">Phase 2 : Symbol Resolution</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>In this stage, the linker resolves symbols using global symbol table
|
|
information to report undefined symbol errors, read archive members, resolve
|
|
weak symbols, etc. The linker is able to do this seamlessly even though it
|
|
does not know the exact content of input LLVM bytecode files because it uses
|
|
symbol information provided by
|
|
<a href="#readllvmobjectfile">readLLVMObjectFile()</a>. If dead code
|
|
stripping is enabled then the linker collects the list of live symbols.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="phase3">Phase 3 : Optimize Bytecode Files</a>
|
|
</div>
|
|
<div class="doc_text">
|
|
<p>After symbol resolution, the linker updates symbol information supplied
|
|
by LLVM bytecode files appropriately. For example, whether certain LLVM
|
|
bytecode supplied symbols are used or not. In the example above, the linker
|
|
reports that <tt>foo2()</tt> is not used anywhere in the program, including
|
|
native <tt>.o</tt> files. This information is used by the LLVM interprocedural
|
|
optimizer. The linker uses <a href="#optimizemodules">optimizeModules()</a>
|
|
and requests an optimized native object file of the LLVM portion of the
|
|
program.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>In this phase, the linker reads optimized a native object file and
|
|
updates the internal global symbol table to reflect any changes. The linker
|
|
also collects information about any changes in use of external symbols by
|
|
LLVM bytecode files. In the examle above, the linker notes that
|
|
<tt>foo4()</tt> is not used any more. If dead code stripping is enabled then
|
|
the linker refreshes the live symbol information appropriately and performs
|
|
dead code stripping.</p>
|
|
<p>After this phase, the linker continues linking as if it never saw LLVM
|
|
bytecode files.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="lto">LLVMlto</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p><tt>LLVMlto</tt> is a dynamic library that is part of the LLVM tools, and
|
|
is intended for use by a linker. <tt>LLVMlto</tt> provides an abstract C++
|
|
interface to use the LLVM interprocedural optimizer without exposing details
|
|
of LLVM's internals. The intention is to keep the interface as stable as
|
|
possible even when the LLVM optimizer continues to evolve.</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="llvmsymbol">LLVMSymbol</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>The <tt>LLVMSymbol</tt> class is used to describe the externally visible
|
|
functions and global variables, defined in LLVM bytecode files, to the linker.
|
|
This includes symbol visibility information. This information is used by
|
|
the linker to do symbol resolution. For example: function <tt>foo2()</tt> is
|
|
defined inside an LLVM bytecode module and it is an externally visible symbol.
|
|
This helps the linker connect the use of <tt>foo2()</tt> in native object
|
|
files with a future definition of the symbol <tt>foo2()</tt>. The linker
|
|
will see the actual definition of <tt>foo2()</tt> when it receives the
|
|
optimized native object file in
|
|
<a href="#phase4">Symbol Resolution after optimization</a> phase. If the
|
|
linker does not find any uses of <tt>foo2()</tt>, it updates LLVMSymbol
|
|
visibility information to notify LLVM intermodular optimizer that it is dead.
|
|
The LLVM intermodular optimizer takes advantage of such information to
|
|
generate better code.</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="readllvmobjectfile">readLLVMObjectFile()</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>The <tt>readLLVMObjectFile()</tt> function is used by the linker to read
|
|
LLVM bytecode files and collect LLVMSymbol information. This routine also
|
|
supplies a list of externally defined symbols that are used by LLVM bytecode
|
|
files. The linker uses this symbol information to do symbol resolution.
|
|
Internally, <a href="#lto">LLVMlto</a> maintains LLVM bytecode modules in
|
|
memory. This function also provides a list of external references used by
|
|
bytecode files.</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="optimizemodules">optimizeModules()</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>The linker invokes <tt>optimizeModules</tt> to optimize already read
|
|
LLVM bytecode files by applying LLVM intermodular optimization techniques.
|
|
This function runs the LLVM intermodular optimizer and generates native
|
|
object code as <tt>.o</tt> files at the name and location provided by the
|
|
linker.</p>
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="gettargettriple">getTargetTriple()</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>The linker may use <tt>getTargetTriple()</tt> to query target architecture
|
|
while validating LLVM bytecode file.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="debug">Debugging Information</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p><tt> ... To be completed ... </tt></p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
Devang Patel<br>
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|